1. Field of the Invention
This invention relates to Karman vortex flowmeters in which fluid flow rate is determined from the frequency at which vortices are formed by a cylindrical obstruction in the flow; more particularly, it relates to such vortex flowmeters having a signal waveform shaper circuit for obtaining a pulse-shaped vortex frequency signal from which the effects of the noise and the beat frequency components superimposed on the vortex frequency are removed.
2. Description of the Prior Art
In the control of internal combustion engines of automobiles, it is essential to measure the amount of air intake accurately and reliably. Among the many types of flowmeters now in use, vortex flowmeters, more particularly Karman vortex flowmeters, are particularly suited for use in the internal cumbustion engines of an automobile for measuring the flow rate of the air intake. Since they have no movable mechanical parts, they are robust and are capable of measuring the flow rate accuratly and reliably under severe and variable conditions; further, they are capable, in principle, of measuring a wide range of flow rates.
The principle of the Karman vortex flowmeter is well known: a double row of line vortices, known as the Karman vortex street, is shed in a flow of a fluid in the wake of a cylindrical obstruction; the vortices, or eddies, are shed periodically first from one side, and then from the other side, of the cylindrical body, wherein the frequency of vortex formation is proportional to the velocity of the fluid flow. Thus, the flow rate of the fluid can be determined from the frequency of vortex formation. The frequency of vortex formation itself may be determined by utilizing sensors such as a thermistor, a strain gauge, or a piezoelectric element; alternatively, it may be determined by utilizing the fact that ultrasonic waves traversing the vortices undergo phase modulation.
A conventional Karman vortex flowmeter utilizing ultrasonic wave beams for determination of vortex generation frequency is disclosed, for example, in the Japanese Patent Publication No. 58-5641. Referring now to FIG. 1 of the drawings, the vortex flowmeter for use in an internal combustion engine disclosed in the above Japanese patent is described. (FIG. 1 is a block diagram of a vortex flowmeter according to this invention, and not that of a conventional flowmeter; however, since the flowmeter of FIG. 1 according to an embodiment of this invention incorporates as vortex signal detector means 100 the flowmeter disclosed in the above mentioned patent, the conventional vortex flowmeter is described in reference to FIG. 1.)
The conventional vortex flowmeter disclosed in the above Japanese patent may roughly be divided into two portions: the first portion, including an ultrasonic oscillator and an ultrasonic receiver, effects phase modulation of the carrier frequency by means of the vortices generated at a period proportional to the velocity of the air flow; on the other hand, the second portion effects demodulation on the phase-modulated signal outputted from the first portion, i.e. removes the carrier frequency from the output signal of the first portion and restores the modulating signal (i.e. the vortex generation frequency signal).
The above first portion has the following organization. Namely, as shown in FIG. 1, an ultrasonic oscillator 4 and an ultrasonic receiver 5 are disposed to oppose other across an air flow passage 1 in the downstream side of a cylindrical obstruction body or a vortex generator 2 disposed in the passage 1 at right angles with the direction of the air flow. The ultrasonic oscillator 4 is oscillated by an ultrasonic oscillation circuit 6 in such a way that the ultrasonic wave is propagated across the Karman vortex street 3 shed in the wake of the vortex generator 2. The ultrasonic wave beam traversing the Karman vortex street 3 undergoes a phase modulation due to the vortices of the Karman vortex street 3 which pass periodically across the ultrasonic beam. Since the frequency of vortex formation is proportional to the velocity of the air flow in the passage 1, the ultrasonic wave is phase-modulated with a modulation frequency proportional to the flow rate. The ultrasonic beam thus undergoing phase modulation is received by the ultrasonic receiver 5. Thereafter, the signal received by the receiver 5 undergoes waveform shaping at the waveform shaper circuit 8 to be outputted therefrom to the phase comparator 9.
The above-mentioned second portion, on the other hand, comprises a phase synchronization loop for demodulating the output signal of the waveform shaper circuit 8. The phase synchronization loop consists of the phase comparator 9, the ultrasonic oscillation circuit 6, the voltage-controlled phase shift circuit 7, and the loop filter (i.e. a low-pass filter which passes the direct-current (dc) component but removes the alternating-current (ac) components from the modulated signal) 10. This phase synchronization loop makes the phase of the output signal of the phase shift circuit 7 follow exactly the phase of the carrier signal of the output of the waveform shaper circuit 8 in the following manner: the output of the ultrasonic oscillation circuit 6 oscillating the ultrasonic oscillator 4 is supplied to the voltage-controlled phase shift circuit 7; the phase shift circuit 7 shifts the phase angle of the output of the ultrasonic oscillation circuit 6 in response to the output voltage signal from the loop filter 10, maintaining the high frequency of the ultrasonic oscillation frequency signal, and outputs the resulting signal to the phase comparator 9; the phase comparator 9 compares the phase of the output of the waveform shaper circuit 8 and that of the output of the voltage-controlled phase shift circuit 7, and the result of the comparison, i.e. a signal which consists of a variey of frequency components including a dc term, but the instantaneous magnitude of which is essentially proportional to the difference in phase between the outputs of the phase shift circuit 7 and the waveform shaper circuit 8, is applied to the loop filter 10. The loop filter 10 removes frequency components other than the dc term the magnitude of which is proportional to the difference in phase between the output of the phase shift circuit 7 and the carrier wave of the phase-modulated signal outputted from the waveform shaper circuit 8. The phase shift angle of the phase shift circuit 7 is controlled in accordance with the dc control voltage outputted from the loop filter 10. Thus, the output of the phase shift circuit 7 is kept exactly in phase with the carrier signal of the output of the waveform shaper circuit 8. The output of the phase comparator 9 is also supplied to the low-pass filter 11 which removes the carrier frequency and passes the modulating signal (i.e. the vortex frequency signal which primarily consists of the frequency component corresponding to the vortex generation frequency). Thus, the low-pass filter 11 outputs a demodulated (sinusoidally varying) vortex generation frequency signal.
The above described vortex flowmeter, however, has the following disadvantage. Namely, in spite of the provision of the waveform shaper circuit 8 and the low-pass filter 11, the output signal of the flowmeter (i.e. the output of the low-pass filter 11) comprises noises, i.e. frequency components other than the vortex generation frequency. These noises include high frequency noises which are predominant under low flow rate conditions, and low frequency noises, called beats, which are predominant under high flow rate conditions. Further, there is a particular type of low frequency noise due to the pulsation of the air flow caused by certain operative conditions of the engine. These noises may become so conspicuous as to disturb the accurate determination of the vortex generation frequency. Since the noises include both high and low frequency components as described above, simple provision of a low-pass or a high-pass filter which effectively suppresses high or low frequency components does not bring about any appreciable improvement.
The above disadvantage is shared by the types of vortex flowmeters other than those utilizing an ultrasonic beam for vortex frequency detection. Thus, Japanese Utility Model Publication No. 59-18332, for example, proposes, in the case of the type of vortex flowmeter utilizing a piezoelectric element for detecting the vortex frequency, a provision of a low-pass filter wherein the filtering function is halted when the vortex frequency exceeds a predetermined level. On the other hand, Japanese Patent Publication No. 58-15045 proposes, in the case of a flowmeter utilizing an ultrasonic beam for detecting the vortex frequency, the provision of a filter of variable pass band which is controlled in response to the information indicating the operative conditions of the engine.
Although the measures proposed by the above Japanese utility model and patent publications may bring some improvement over the more conventional vortex flowmeters, there still remains much to be hoped for with respect to the effective removal of noises and the accurate and reliable restoration of the vortex frequency signal.
The last-named Japanese patent publication further proposes a provision of a Schmitt trigger circuit for converting the vortex frequency signal into a pulse-shaped electric signal, wherein the trigger level or the amount of hysteresis of the Schmitt trigger is altered in response to the information indicating the operative conditions of the engine. This measure may bring about an improvement over conventional waveform shaper circuits, but it is not enough to provide a circuit which is capable of obtaining a pulse-shaped signal having a frequency that infallibly corresponds to the vortex generation frequency.